The invention relates to a cell for the production of aluminum by the electrolysis of an aluminum compound dissolved in a molten electrolyte, for example alumina dissolved in a molten fluoride-based electrolyte. It concerns in particular a cell of advanced design having a cathode of drained configuration, and a non-carbon anode facing the cathode both covered by the molten electrolyte.
The invention also relates to methods of operating the cells to produce aluminum.
The technology for the production of aluminum by the electrolysis of alumina, dissolved in molten cryolite-based electrolyte and operating at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminum that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite-graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode and to which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks. The side walls are also covered with prebaked anthracite-graphite carbon plates or silicon carbide plates.
Conventional aluminum production cells are constructed so that in operation a crust of solidified molten electrolyte forms around the inside of the cell sidewalls. At the top of the cell sidewalls, this crust is extended by a ledge of solidified electrolyte which projects inwards over the top of the molten electrolyte. The solid crust in fact extends over the top of the molten electrolyte between the carbon anodes. To replenish the molten electrolyte with alumina in order to compensate for depletion during electrolysis, this crust is broken periodically at selected locations by means of a crust breaker, fresh alumina being fed through the hole in the crust.
This crust/ledge of solidified electrolyte forms part of the cell""s heat dissipation system in view of the need to keep the cell in continuous operation despite changes in operating conditions, as when anodes are replaced, or due to damage/wear to the sidewalls, or due to over-heating or cooling as a result of fluctuations in the operating conditions. In conventional cells, the crust is used as a means for automatically maintaining a satisfactory thermal balance, because the crust/ledge thickness self-adjusts to compensate for thermic unbalances. If the cell overheats, the crust dissolves partly thereby reducing the thermic insulation, so that more heat is dissipated leading to cooling of the cell contents. On the other hand, if the cell cools the crust thickens which increases the thermic insulation, so that less heat is dissipated, leading to heating of the cell contents.
The presence of a crust of solidified electrolyte is considered to be important to achieve satisfactory operation of commercial cells for the production of aluminum on a large scale. In fact, the heat balance is one of the major concerns of cell design and energy consumption, since only about 25% of such energy is used for the production of aluminum. Optimization of the heat balance is needed to keep the proper bath temperature and heat flow to maintain a frozen electrolyte layer (side ledge) with a proper thickness.
Considerations concerning the refractory and insulating materials used in conventional cells to control the heat flow are discussed in the monograph xe2x80x9cMaterials Used in the Hall-Heroult Cell for Aluminum Productionxe2x80x9d by H. Zhang. V. de Nora and J. A. Sekhar, published by The Minerals, Metals and Materials Society, Pennsylvania, USA, 1994, see especially Chapter 6.
In conventional cells, the major heat losses occur at the sidewalls, the current collector bars and the cathode bottom, which account for 35%, 8% and 7% of the total heat losses respectively, and considerable attention is paid to providing a correct balance of these losses.
Further losses of 33% occur via the carbon anodes, 10% via the crust and 7% via the deck on the cell sides. This high loss via the anodes is considered a inherent in providing the required thermal gradient through the anodes.
In the literature, there have been suggestions for cells operating with non-carbon anodes with or without a crust of solidified electrolyte, but so far none of these designs has proven to be feasible. Previously this was due principally to the difficulties encountered in developing anode materials that remained sufficiently stable in the aggressive environment.
However, even with available promising non-carbon anode materials such as those based on nickel-iron-aluminum or nickel-iron-aluminum-copper with an oxide surface as described in U.S. Pat. No. 5,510,008 (de Nora et al), there is still a need to provide a redesigned cell of advanced design in order to achieve the potential advantages of the oxygen-evolving anode materials on the one hand and of the drained cathode configuration on the other hand, and to improve the overall cell efficiency.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for a cell operating with non-carbon anodes that can operate without crust formation and which also facilitate the implementation of a drained cathode configuration.
One object of the invention is to provide an aluminum production cell of advanced design incorporating non-carbon oxygen-evolving anodes which is a efficient in operation and can operate without formation of a crust of frozen electrolyte.
Another object of the invention is to provide an aluminum production cell of advanced design wherein the cell efficiency is improved by better control of the thermic losses associated with the anodically-evolved gases.
Another object of the invention is to permit more efficient cell operation by improving the distribution of electric current to the cathode cooperating with non-carbon oxygen evolving anodes.
A further object of the invention is to provide a cell of advanced design with a non-carbon anode in combination with novel cathode which has improved distribution of electric current and can be easily produced and fitted in the cell, and which simplifies dismantling of the cell to replace or refurbish the cathodes.
A yet further object of the invention is to provide a cell of advanced design which facilitates the implementation of a drained cell configuration.
Yet another object of the invention is to provide a cell of advanced design which combines the advantages of a drained cathode configuration and of non-carbon oxygen evolving anodes, is thermally efficient, easy to construct and service, and efficient in operation.
A yet further object of the invention is to provide a cell of advanced design enabling drained cathode operation where ease of removal of the anodically produced gases is combined with ease of collection of the product aluminum.
An even further object of the invention is to provide an aluminum production cell in which fluctuating electric currents that produce a variable electromagnetic field are reduced or eliminated thereby reducing or eliminating the adverse effects that lead to a reduction of the cell efficiency.
One main aspect of the invention concerns a cell of advanced design for the production of aluminum by the electrolysis of an aluminum compound dissolved in a molten electrolyte, having a cathode of drained configuration and at least one non-carbon anode facing the cathode. Both the cathode and the anode are covered by the electrolyte. In accordance with the invention, the upper part of the cell contains a removable thermic insulating cover placed just above the level of the electrolyte.
Thanks to this removable thermic insulating cover, heat losses from the anodically-evolving gases are drastically reduced, enabling the cell to operate without a frozen top crust of molten electrolyte. Moreover, removal of the anodes for servicing is simple, by removing the entire thermic insulating cover, or by removing sections of the cover associated with the individual anodes or groups of anodes.
The cathode advantageously comprises a cathode mass supported by a cathode carrier made of electrically conductive material which serves also for the uniform distribution of electric current to the cathode mass from current feeders which connect the cathode carrier to the negative busbars. The entire cathode is contained in an outer structure from which it is separated electrically and thermically. Further details of this advantageous arrangement are described in applicant""s corresponding international patent application PCT/IB-97/00589.
The advanced-design cell preferably has a cell outer structure which has a top cover for additional thermic insulation and collection of the evolved gases. This top cover encloses the removable thermic insulating cover placed just above the level of the electrolyte, and both covers have passages for feeding alumina and for the exit of the evolved gases during electrolysis.
The above-mentioned cathode carrier is usually an inner metal shell or plate. In some embodiments, the inner metal shell extends substantially to the top of the cell side walls.
Usually, the active part of the non-carbon anode is covered completely by the molten electrolyte, only the anode current feeder remaining above the electrolyte. The non-carbon anode can be located above the cathode, the anode and cathode having facing horizontal surfaces, or having facing surfaces inclined to horizontal. Alternatively, the non-carbon anode has vertical or inclined active parts interleaved with corresponding vertical or inclined cathode surfaces.
In nearly all cases, the cathode will most advantageously operate as a drained cathode, though it is possible also to operate with a shallow pool of molten aluminum.
The advanced-design cell can have a removable thermic insulating cover fitting over all of the anodes, or fitting over a group of anodes. This thermic insulating cover can be removed entirely or by sections for replacement or servicing of one or more of the non-carbon oxygen-evolving anodes which are non-consumable or substantially non-consumable.
In another design, each anode is fitted with a thermic insulating cover removable with its anode. In this case, the thermic insulating covers of adjacent anodes can be arranged to fit together when the anodes are immersed in the molten electrolyte, to form a thermic insulating cover over several or all of the anodes. Also in this case, when an anode has to be removed and replaced or serviced, it can be removed with its cover, and a new or refurbished anode fitted with a cover can be inserted in place of the removed one.
As described further in the applicant""s international patent application PCT/IB97/00589, the cathode of the advanced-design cell advantageously *comprises a cathode mass made mainly of an electrically conductive non-carbon material or made of a composite non-carbon material composed of an electrically conductive material and an electrically non-conductive material. This non-conductive material can be alumina, cryolite, or other refractory oxides, nitrides, carbides or combinations thereof.
The conductive material of the cathode can include at least one metal from Groups IIA, IIB, IIIA, IIIB, IVB, VB and the Lanthanide series of the Periodic Table, in particular aluminum, titanium, zinc, magnesium, niobium, yttrium and cerium, and alloys and intermetallic compounds thereof.
In any event, the bonding metal of the composite material usually has a melting point from 650xc2x0 C. to 970xc2x0 C. For instance, the composite material is advantageously a mass made of alumina and aluminum or an aluminum alloy, see U.S. Pat. No. 4,650,552 (de Nora et al), or a mass made of alumina, titanium diboride and aluminum or an aluminum alloy.
The composite material can also be obtained by reaction such as that utilizing, as reactants, TiO2, B2O3 and Al.
The cathode mass can alternatively be made mainly of carbonaceous material, such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Pat. No. 5,362,366 (Sekhar et al), prebaked carbon blocks assembled together on the shell, or graphite blocks, plates or tiles.
The cathode mass is preferably impervious to, or is made impervious to, molten aluminum and to the molten electrolyte.
To operate as a drained cathode, or with a shallow pool of molten aluminum, the cathode""s active surface, usually its upper active surface, is aluminum-wettable, for example the upper surface of the cathode mass is coated with a coating of refractory aluminum wettable material such as slurry-applied titanium Lu diboride as described in U.S. Pat. No. 5,316,718 (Sekhar et al). Also, where the cathode has an inner metal cathode carrier shell or plate, its upper surface in contact with the cathode mass can be coated with a coating of refractory aluminum-wettable material or other protective materials.
Advantageously, the surface of the cathode mass is maintained at a temperature corresponding to a paste state of the electrolyte whereby the cathode mass is protected from chemical attack. For example, when the cryolite-based electrolyte is at about 950xc2x0 C., the surface of the cathode mass can be cooled by about 30xc2x0 C., whereby the electrolyte contacting the cathode surface forms a viscous paste which protects the cathode surface. The surface of the cathode mass can be maintained at the selected temperature by supplying gas via an air or gas space between the cathode holder and the electric and thermic insulating mass.
The anodes are preferably made principally of nickel-iron-aluminum or nickel-iron-aluminum-copper with an oxide surface. For example, the anodes are a reaction product of a powder mixture of nickel-iron-aluminum or nickel-iron-aluminum-copper, as described in U.S. Pat. No. 5,510,008 (de Nora et al). In use, the anodes can be protected by an in-situ formed or maintained protective coating of cerium oxyfluoride, as described in U.S. Pat. No. 4,614,569 (Duruz et al).
When an anode must be changed during operation, it can be removed with its associated section of the thermic insulating cover and replaced with a new anode fitted with the same section of the insulating cover or with its own thermic insulating cover.
It is advantageous to preheat each non-carbon anode before it is installed in the cell during operation, in replacement of an anode that has become disactivated or requires servicing. By preheating the anodes, disturbances in cell operation due to local cooling are avoided such as the formation of an electrolyte crust whereby part of the anode is not active until the electrolyte crust has melted.